Because of their properties, photocurable resin compositions are widely used for many purposes. For instance, photocurable resin compositions are stable at ambient temperature if they are not exposed to light. In addition, cured articles can be prepared therefrom in a short period of time by exposure to light without using heat. Accordingly, photocurable resin compositions, both solid and liquid, are widely used in paint, ink, coatings, insulation, relief printing plates and resist materials for the preparation of printed circuit boards.
The preparation of a liquid photocurable resin composition is relatively easy. However, when it is used as photoresist for the preparation of printed circuit boards, for example, the resist-coated boards cannot be easily stacked and stored because of the resulting liquid, tacky resist coating on the boards. For the same reason, a photo-imaging mask cannot be directly placed in contact with the resin composition. Further, when a liquid resist is used as potting material for electronic parts of components, specific amounts of the liquid photocurable resin composition have to be weighed and applied to an electronic part or component in a mold prior to irradiating and curing the composition with light. Such steps are laborious and uneconomical.
In addition, if the mold used is non-transparent metal, only the area of resist directly under or adjacent to the mold's aperture can be cured. Thus, the size and shape of the electronic part or component which can be used will be limited.
To avoid problems associated with liquid photocurable compositions, solid compositions are sometimes used. Prior to forming those solid compositions, the photocurable resin is dissolved in solvent which is removed prior to photocuring. For example, when using solid compositions as a photoresist for the production of printed circuit boards, the solution of resist is directly coated onto a copper clad laminate and then dried by removing solvent. In the alternative, it may be coated and dried on a temporary substrate such as a polyester film to form a film-like resist, which is then transferred to the copper clad laminate. However, removal of the solvent not only takes more time than methods using liquid resists, but also the evaporation of solvent during the removal causes environmental problems. Even further, defects caused by air bubbles appear in the resist during the drying and transferring steps. Furthermore, when a relatively thick and uniform sheet material such as a photosensitive sheet for letterpress printing is required, it is fairly difficult to economically remove the solvent and dry the sheet without formation of bubbles. It is also difficult to obtain a sheet having a uniform thickness due to shrinkage, surface tension, and hot air which is typically used in the drying process. As a result of these difficulties special and expensive apparatus are required. Moreover, a photocurable resin composition which contains a solvent is generally not suitable for potting electronic parts or components which usually have a complicated shape.
To avoid these flaws and problems, especially in the area of solid photocurable sheets for letterpress printing, efforts have been made to prepare a photocurable resin composition which can be melted and cast without using any solvent. See Japanese Patent Publication (Kokai) Nos. SHO 62-138845 (1987), SHO 63-8648 (1988) and HEI 1-131548 (1989). However, such photocurable compositions are inherently and eventually thermally curable. As a result, innovative resin formulations and apparatus are necessary in order to melt and cast such compositions continuously and steadily at elevated temperatures. Further, even if one can obtain a composition which is solid at ambient temperature and which possesses sufficient photosensitivity at ambient temperature, as well as sufficient stability in viscosity at high temperatures to allow melting and casting, a relatively longer period of time is still needed to solidify the melted composition after it leaves a nozzle at ambient temperature. As a result, special techniques would be required to obtain a sheet having the desired thickness.
Another problem occurs when potting electronic parts or components with these resin compositions. Because of the compositions viscosities, they do not defoam easily. Thus, longer periods of time will be required to allow the compositions to solidify prior to being released from the mold.
Recently, in the area of photoimageable resists used in the manufacture of printed circuit boards or photocurable sheets for letterpress printing, efforts have focused on eliminating conventional organic solvents used to develop such resists. The aforementioned environmental reasons is a chief reason for such efforts. As a result, photocurable resin compositions developable with water or an aqueous alkaline solution have been developed and have been put to practical use. However, to obtain such water developable compositions other properties and advantages are compromised. For example, while it is desirable to have solder resists and letterpress printing plates which are aqueous developable, a high level of water resistance is still required for the images obtained after their development. In order to fulfil the latter requirement, liquid photocurable resin compositions are usually used because they are generally developable in water. However, as mentioned earlier, controlling the thickness of a coating prepared from such liquid photocurable resin compositions is difficult during a coating process. As mentioned earlier, obtaining precise images from such liquid coated articles is difficult because a photo mask cannot directly be placed in contact with the surface of liquid resin during exposure to light.
On the other hand, improvements have been made to solid photocurable resin compositions which can fulfil both of the requirements mentioned above. However, the manufacturing process, development process and post-treatment process used with those solid photocurable resin compositions are more complicated and time-consuming. Even further, treatment of the aqueous waste produced thereby still remains as a problem to be solved. Accordingly, the art in the technical areas mentioned above all still searching for a solventless photocurable composition which avoids the problems detailed above.
It is known that a mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer which can form a polymer complex. Several industrial applications of such polymer complexes have been proposed. For example, radiation degradable polymer mixtures of an isotactic member and a syndiotactic member for photoimaging are disclosed in Japanese Patent Publication Nos. (Kokoku) SHO 53-40615 (1978) and SHO 55-28054 (1980) and Japanese Patent Publication (Kokai) No. SHO 62-254142 (1977).
Further, gelled substances formed from a polymer complex of an isotactic polymethyl methacrylate and a syndiotactic polymethyl methacrylate in a solvent are proposed for use in organic electrolyte batteries [Japanese Patent Publication (Kokai) No. SHO 55-21862 (1980)]; for anti-blood coagulants [Japanese Patent Publication (Kokai) SHO 48-55946 (1973)]; and for semipermeable membranes or separatory membranes [Japanese Patent Publication (Kokai) No. SHO 50-128773 (1975)]. Compositions comprising a polymer complex of isotactic polymethyl methacrylate and syndiotactic polymethyl methacrylate in a solvent, wherein the composition reversibly and quickly undergoes gel/sol transition around the gel melting point are also known. Journal of Applied Polymer Science, Volume 20, pp. 1775-1785 (1976). A glass laminate prepared by dissolving an isotactic polymethyl methacrylate and a syndiotactic polymethyl methacrylate in polymerizable monofunctional monomers, pouring the solution in the space between two glass plates and polymerizing the monomers is also known. See Japanese Patent Publication (Kokuku) No. SHO 47-14834 (1972).
It has now been found that a variety of photopolymerizable ethylenically unsaturated compounds can dissolve polymer compositions having an isotactic polymethyl methacrylate moiety and a syndiotactic polymethyl methacrylate moiety, wherein these polymer compositions give a photocurable self-retainable gel capable of quickly and reversibly undergoing gel/sol transition by heating at a temperature above its flow temperature or cooling to a temperature below its flow temperature. In addition, the gel can easily be formed into a photocurable self-retainable shaped article, especially shaped articles, free of air bubbles in the absence of any additional solvent.